The present invention relates to a process and apparatus for uniform and reproducible impregnation of fixed bed catalyst supports in the form of loose particles with a desired shell volume, by applying atomized impregnating solutions of precursors of the catalytically active components onto the catalyst supports that are circulated in a tank.
Particulate catalysts are used for various catalytic processes. Catalyst supports in the form of extruded strands, granulates or pellets, are used in this case. Their geometric dimensions are in the range of 1 to 10 mm. In order to absorb the catalytically active components, they have a high pore volume--generally 0.5 to 2 ml/g--and a large specific surface (BET surface area determined by nitrogen adsorption according to DIN 66132). Depending on the catalytic process, homogeneous, ring-shaped or shell-like distribution profiles of the catalytically active components are formed through the cross-section of the catalyst support.
Catalysts impregnated with ring-shaped or shell-like distribution profiles are required for selective processes, in which prolonged contact of the reactants inside the support in the presence of the catalyst would lead to undesirable secondary reactions. A typical example of this is vinyl acetate synthesis.
Oxidic materials with a high surface area such as, for example, silicon oxide, aluminum oxides of various crystallographic forms, titanium oxide in anatase and/or rutile forms, zirconium oxide, zeolites as well as pellets containing carbon, either alone or in mixtures, are generally used as catalyst supports. These and other inorganic oxide substances known in the art can be used as the catalyst supports according to this invention. The lattice structure of the oxidic support materials can be stabilized to increase their temperature stability, for example, by doping with rare earths. Depending on the required catalytic function, these materials are impregnated with base metal components (e.g. nickel, copper or iron), noble metal components (e.g. platinum, palladium or rhodium), or combinations of these components. Precursors of these active components in the form of chlorides or nitrates are used for this purpose as known in the art.
Flooding methods, the incipient wetness method and spraying methods, with and without pre-coating, are known for the impregnation of catalyst supports. The suitability of these processes varies for the different distribution profiles.
The distribution profiles of the catalytically active components across the cross-section of the catalyst supports is dependent on the selected impregnation process. Moreover, the resulting diffusion profiles of the active components are influenced by chromatographic effects, i.e. they depend on the choice of support material as well as on the type of precursor of the catalytically active components in each case and the solvent used. Different diffusion profiles are produced in the same support material for one and the same catalytic component, depending on the precursor used (e.g. chlorides or nitrates). Moreover, as described in DE-PS 25 31 770, the diffusion profiles may be further influenced by pre-coating with various organic liquids.
Also essential for the finally adjusted profile is the time at which the catalytically active components are fixed by drying and calcination of the impregnated support or reduction of the precursors of the catalytic components. The shorter the interval between impregnation and fixing, the more likely the catalytic components will be fixed close to the surface in the form of a shell.
In the flooding process, the catalyst supports are immersed in an excess of impregnating solution, are removed from the solution after a certain period of impregnation, dried and then optionally activated. In this type of impregnation, the precursors of the catalyst diffuse deeply into the catalyst support. Depending on the duration of impregnation and on the combination of support-material/catalytic-precursor, a more or less uniform distribution of the catalytically active components results. For processes, in which the catalytic conversion occurs essentially on the surface of catalyst supports, this means that valuable catalytically active material is wasted deep in the catalyst supports. In addition, this may lead to undesirable secondary reactions, if, by diffusing into the catalyst supports, the reactants come into contact with the catalytically active components for a long period of time before they diffuse out of the supports once more.
The quantity of impregnating solution absorbed by an individual support particle in the flooding process depends on the individual absorption capacity of each support particle. Since the absorption capacity of the support particles fluctuates from particle to particle, even when the support particles are mass-produced, the support particles, as a consequence, are laden with different quantities of catalytically active components.
In the incipient wetness process, the maximum absorption capacity of the catalyst supports with respect to the impregnating liquid is determined first. The soluble precursors of the catalyst components are then dissolved in a quantity of impregnating liquid, which corresponds to 80 to 95% of the absorption capacity of the given quantity of catalyst supports. The quantity of the dissolved catalyst components is matched to the desired loading of the given quantity of catalyst supports with the catalytically active components. The impregnating solution is dispersed as evenly as possible over the catalyst supports circulated in a coating tank. This procedure ensures that the catalyst supports are coated with the given quantity of active components. However, this is only averaged out over a large number of the supports. Loading of the individual support particles can, however, differ from particle to particle because of the difference in absorption capacity, as in the case of the flooding process.
In the case of spray impregnation, a specific quantity of impregnating solution is sprayed through single- or dual-material nozzles over the catalyst supports circulated in a tank. The spraying method is particularly suited to the production of catalysts on a larger scale. With the spraying method the catalyst supports may be impregnated equally well in batches or in a continuous process. However, it has not been possible hitherto to produce shell catalysts with a high degree of uniformity using this process. A lack of uniformity may also be observed in this process, in particular in the impregnation over the surface of an individual catalyst support.
The object of the invention is therefore to provide an improved spraying process, which enables the production of shell catalysts with a high degree of uniformity and desired shell volumes without pre-coating the support.